Mixing apparatus

ABSTRACT

A process and apparatus for the mixing of material by means of the combination of sheer-dispersion and/or extensional-dispersion and distributive mixing actions, in which the mixing occurs in one or more stages within stress inducing flow channels between movable members whereby the material is essentially propelled through the flow channels of such stages by pumping actions provided by the relative movement between the members within the mixer itself.

The present invention relates to a mixing apparatus.

The operation of mixing is generally understood to comprise two distinctactions; dispersive mixing and distributive mixing. In dispersive mixingthe individual parts of the materials being mixed, whether solid orfluid, have their respective geometries altered by means of appliedstresses. This usually takes the form of reducing the average size ofindividual parts while increasing their numbers. In distributive mixingthe individual parts of the materials, whether solid or fluid, areblended together in order to obtain a spatial uniformity in thedistribution of the various material parts with respect to one another.A good mixing operation thus usually requires both dispersive anddistributive mixing actions to occur.

Distributive mixing is primarily a function of the geometry of themixing apparatus and known mixers typically fall into two general typesproviding either random or structured distributive mixing. Randomdistributive mixers achieve mixing by randomly agitating the materialsand include known mixers such as tumble-blenders and ribbon-blenders.Structured-distributive mixers on the other hand achieve mixing bysystematically repeating a geometrically controlled sequence ofdividing, reorienting and rejoining the materials and include staticmixers and cavity transfer mixers.

In contrast, dispersive mixing is primarily a function of forces,pressures, stresses and strains applied to the materials. In general,the size reduction of materials that is required in dispersive mixing isachieved by applying stresses to the materials. These applied stressesusually take the form of compressive, tensile or shear stresses. Formixing fluid materials the predominant method of stressing has been bymeans of applying shear, as this can readily be achieved by utilisingthe drag forces that exist within a fluid bounded by two relativelymoving surfaces in a machine. Examples of such mixers include internalrotor/stator mixers in which the material is sheared between the rotorand the stator surfaces. Shear stressing can also be obtained by forcinga fluid material over one or more surfaces that do not have a motionrelative to one another, for instance between the walls of a channel. Inthis case it is still possible to generate significant shear stresses inthe fluid, but only at the expense of providing some form of pumpingenergy to propel the fluid over the surfaces. It has long beenrecognised however that an alternative mechanism, that of extensionalflow, is capable of subjecting fluid materials to compressive andtensile stresses that in practice can be much higher than the shearstresses.

Extensional flow requires that the fluid be pressurised in order topropel it between surfaces that subject the fluid to tensile orcompressive stresses. Such surfaces can be generally orientated in thedirection of the flow in which case the flowing material is acceleratedor decelerated along its flowpath by virtue of mass conservation, orgenerally orientated across the direction of the flow, in which case theflowing material is decelerated and thus compressed by virtue of thechange in the momentum of the fluid, such as in impact. Known mixersdesigned to operate on the basis of extensional flows for dispersionhave thus required external means of pressurisation in the form ofhigh-pressure pumps located upstream (the same requirement for pumpingapplies to a mixer operating on the basis of shear flow betweennon-moving surfaces as mentioned above). Given that it is often arequirement that any given part of the material being mixed is subjectedto a number of stressing cycles it is apparent that the overallpressures required to provide extensional flows and shear flows througha mixer can become prohibitively high. Additionally, the need toengineer such a mixer so as to ensure that the extensional flow andshear flow occur with maximum efficiency, i.e. the minimum pressureloss, is relatively costly.

It is an object of the present invention to provide a mixing apparatuswhich obviates or mitigates the above disadvantages.

According to the present invention there is provided a mixing apparatusfor mixing a material, the apparatus comprising one or more flowchannels and at least two members which are either eccentrically mountedone within the other so as to define a chamber therebetween, or axiallymounted defining a chamber between facing surfaces thereof, and whichare rotateable relative to one another to thereby produce a pumpingforce to force material through said flow channels and chamber tothereby subject the material to stresses within said flow channelsand/or said chamber that result in extensional-dispersive and/orshear-dispersive mixing.

Preferably the apparatus is further adapted to subject the material todistributive mixing.

The mixer preferably comprises a plurality of said stress-inducing flowchannels in at least two sets defined by respective channel membersarranged such that material is pumped from channels of one set tochannels of another.

Pumping force may be imparted to the material during and/or intermediatetwo sets of said stress-inducing flow channels.

The channels may have sides that are parallel, convergent or divergentrelative to one another and any channel may be entirely contained withina single channel defining member of the mixer or alternatively may beformed within the surface of one channel member and bounded by theadjacent surface of any other component of the mixer (e.g. anotherchannel defining member). The channels may be, for instance, radialchannels within generally concentric members or axial channels withinmembers juxtaposed in an axial direction.

Chambers are preferably provided between channel defining members of themixer the chambers providing random-distributive and bothshear-dispersive and extensional-dispersive mixing to the mixingcomponents. The chambers may, for instance, be annular spaces betweenconcentric or eccentric surfaces, or be axial spaces between surfacesthat are parallel or not-parallel. The chambers may be sufficientlysmall so as to permit the channel members to come into contact.

The pumping actions may, for instance, arise from centrifugal forces orfrom drag forces, or may take the form of positive-displacement pumpingsuch as vane pumping, gear pumping or piston pumping.

In preferred embodiments of the invention there is provided means toobtain an amount of backflow mixing, in which the direction of the flowwithin a channel (or chamber between sets of channels) is reversedduring part of the pumping cycle as a result of a reversal in thedirection of the pressure differential across the channel (or chamber).The amount of flow occurring in the reverse direction may be controlledby means of the design of the channel (or chamber), singly or incombination, in which flow in one direction is subjected to a greaterresistance than it is in the opposite direction. In this instance, thechannels (or chamber) can be designed to operate as valves that permitmore flow in one direction than they do in another, while at the sametime being capable of imparting the appropriate mixing actions to thematerials. Alternatively, the amount of flow occurring in the reversedirection may be controlled by means of the design of the pumpingactions in which a greater pumping effect is achieved in one directionthan it is in the other. This backflow can have a beneficial effect inincreasing the residence time within the mixing unit, thereby subjectingany part of the material to an increased number of mixing actions. Insome embodiments of the invention there may be no net flow in any onedirection during mixing so that the mixing operation is essentiallystatic (the mixer could have a common inlet/outlet).

Apparatus in accordance with the present invention can be used to mix asingle material (the term mixing in this context is used throughout themixing industry referring to, for example, dispersive mixing of amaterial to break it down into smaller component parts which may becoupled with distributive mixing in distributing those smaller partsthrough the material as a whole) or a number of different materialsincluding mixtures of fluids and solids, or indeed just solids which arecapable of behaving in a manner analogous to fluids.

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 is a sectional side-elevation of a mixing apparatus in accordancewith a first embodiment of the present invention;

FIG. 2 is a sectional end-view of the embodiment of FIG. 1;

FIGS. 3a, 3 b, 3 c, 3 d, 3 e and 3 f are illustrations to an enlargedscale of various alternative types of channel formation;

FIG. 4 is a section side-elevation illustrating a modification to themixing apparatus of FIG. 1.

FIG. 5 is a sectional side-elevation of a third embodiment of thepresent invention; and

FIG. 6 is a sectional end-view of the mixing apparatus of FIG. 5.

Referring to FIGS. 1 and 2, the illustrated mixer comprises a rotor 1and rotor shaft 2, driven by some external means (not shown), mountedwithin a generally cylindrical housing 3 having an inlet 13 and outlet14. Two fixed stator rings 4, each defining a set of radialstress-inducing channels 5, are mounted on a planar surface 6 supportedwithin the housing 3 and are concentric with and perpendicular to anaxis through point X (see FIG. 2). The rotor 1 comprises a single rotorring 7 defining a set of radial stress-inducing channels 8 which isconcentric with the rotor shaft 2 which has its axis through a point Y(see FIG. 2). The rotor ring 7 is supported on a planar surface 9 whichis perpendicular to the rotor axis.

The axis of rotation of the rotor 1 is parallel to the axis ofconcentricity of the stator rings 4 and is offset from it by a distanceXY, with the result that the rotor ring 7 rotates with an eccentricityrelative to the stator rings 4. The rotor ring 7 carries a number ofvanes 10 mounted between the outer surface of the inner stator ring 4and the inner surface of the outer stator ring 4. The vanes 10 arecapable of sliding radially with respect to the rotor ring 7 andcircumferentially with respect to the stator rings 4 and extend axiallyto slide against the planar surface 9 of the rotor on one side and theplanar surface 6 of the stator on the other side.

The combination of the surfaces of rotor ring 7, rotor planar surface 9,stator rings 4, stator planar surface 6 and vanes 10 serves to enclose aset of inner and outer compartments 11 on either side of the rotor ring7 respectively within the annular chamber defined between the statorrings 4. That is, two compartments 11 are defined between each pair ofneighbouring vanes 10, an inner compartment 11 between the rotor ring 7and the inner stator ring 4 and an outer compartment 11 between therotor ring 7 and the outer stator ring 4. As the rotor ring 7 rotateseach compartment 11 rotates with it between respective vanes 10 and thevolume of each compartment progressively increases and decreases as itrotates as a consequence of the eccentricity of the rotor ring 7relative to the stator rings 4. A pumping action is thus provided inwhich material is drawn into each compartment 11 as it expands and isexpelled as it contracts. The material enters and exits each compartmentprimarily through the channels 5 and 8 that are radially disposed withinthe adjacent rings, although a controllable amount of material flow cantake place through annular spaces 12 between the rotor ring 7 and thestator planar surface 6 and between the stator rings 4 and the rotorplanar surface 9.

In operation, material to be mixed enters through inlet 13 and is drawnradially through flow channels 5 in the inner stator ring 4 intoexpanding inner compartments 11 defined between the inner stator ring 4and the rotating rotor ring 7. At the same time, contracting innercompartments 11 defined between the inner stator ring 4 and the rotorring 7 pump material radially through the rotor ring flow channels 5into outer compartments 11 defined between the rotor ring 7 and theouter stator ring 4. In addition to the pumping action of contractingouter compartments 1, material will also be drawn through the channels 5as outer compartments 11 defined between the rotor ring 7 and outerstator ring 4 expand. Thus, material flows radially outwards through therotor ring 7 between each pair of inner and outer compartments 11defined between respective pairs of vanes 10 through a combination ofcontraction of the inner compartments 11 and expansion of thecorresponding outer compartments 11. Similarly, as outer compartments 11defined between the rotor ring 7 and the outer stator ring 4 contractmaterial is pumped through channels 5 defined in the outer stator ring 4to the annular part of outlet 14. In this way, material is continuallypumped through the apparatus from inlet 13 to outlet 14 simply byrotation of rotor ring 7.

The cross-sectional areas of each of the channels 5 and 8 illustrated inFIGS. 1 and 2 converge in a radially outwards direction. Thisconvergence within each channel 5/8 imposes extensional stresses andshear stresses on the material contained therein thereby subjecting thematerial to a combination of extensional-dispersive and shear-dispersivemixing. The amount of stressing is related both to the geometry of eachchannel 5 and 8 and to the flowrates arising from the pressuredifferentials imposed across each channel 5 and 8. For instance, thegeometry of the channels can be selected to vary the degree ofextensional and/or shear stressing. For instance, the channels could befigured so that extension stresses are effectively reduced to zero sothat only shear-dispersive mixing occurs within the channels 5 and 8.

In addition to the extensional-dispersive and shear-dispersive mixingprovided by the channels 5 and 8, there is also distributive mixing asthe material passes between the stator rings 4 and rotor ring 7. Thatis, each inner compartment 11 receives material from each channel 5 ofthe inner stator ring in sequence and thus each channel 8 in the rotorring 7 receives material from each channel of the inner stator ring.Moreover, material passing from each outer compartment 11 to the annularpart of the outlet 14 is distributed amongst each of the channels 5 ofthe outer stator ring 4 as the respective compartments 11 rotate. Thus,material entering through inlet 13 is distributed through all channels 5in the inner stator ring, material passing through each channel 5 in theinner stator ring 4 is then distributed amongst all channels of therotor ring 7, and material passing through each channel 8 in the rotorring 7 is distributed amongst all channels 5 of the outer stator ring 4.

There will also be some degree of shear-dispersion occurring within thecompartments 11 by virtue of rotation of the rotor ring 7 relative tothe stator ring 4, and some extensional dispersive mixing as a result ofthe “tapering” geometry of the compartments 11.

In addition, although the net flow through the mixer is from the inletto the outlet 14 as described above, it will be appreciated that as eachcompartment 11 contracts there will be a pumping force both radiallyinward and outward and similarly as each compartment 11 expands it willdraw in material from both radially outer and radially inner parts ofthe mixer. This is also beneficial. In more detail, the material flowthrough each channel 5/8 illustrated in FIGS. 1 and 2 is greater in theradially outward direction than it is in the radially inward directionas a result of the interaction between the geometry of the channels andthe material. This interaction is a function of a number of aspectsincluding material viscosity, material-surface effects, and themagnitude and direction of flow velocities. The radially-outward biasresults in the net flow of material in a radially outward direction,from an inlet 13 to an outlet 14 of the mixer. However, because thematerial is capable, within the geometry illustrated, of also flowingradially-inward during part of each revolution of the rotor 1, an amountof back-mixing is obtained in which the material is subjected to themixing actions in the reverse direction. This back-mixing operationserves to increase the residence time of the material within the mixerand especially to increase the amount of active mixing taking place, asany part of the material passing through the mixer is subjected to morepasses through the mixing elements than would be achieved if totallyefficient pumping were to be used. However, the design illustrated isrequired to achieve a balance between the pumping efficiency required topropel material through the channels in order to achieve the requiredamounts of dispersive mixing, and the pumping inefficiency desired toachieve the required residence time within the mixer.

The directional bias of the material flow through the mixer illustratedin FIGS. 1 and 2 can therefore be affected significantly by the designof the channels. FIGS. 3a to 3 f illustrate some alternative channeldesigns, in which the direction of the material flow is required to bepredominantly upwards in the direction of the arrows shown. FIG. 3ashows a radially convergent channel 15 of the type illustrated in FIGS.1 and 2. FIG. 3b shows a slanted channel 16 in which the direction ofrotation of the disk affects the directionality of flow within thechannel. FIG. 3c shows a radially convergent channel 17 in which thesurface area of the inner end is larger than that of the outer end,thereby imposing a greater resistance to flow in one direction than inthe other. FIG. 3d shows a pair of radially convergent/divergentchannels 18 in which the surface area of the inner end is larger thanthat of the outer end, thereby imposing a greater resistance to flow inone direction than in the other. FIG. 3e shows a radially convergentchannel 19 with a slant in which the direction of rotation of the diskaffects the directionality of flow within the channel. FIG. 3f shows achannel 20 with a spring-loaded ball valve 20 in which the ball seatsagainst an orifice to prevent flow in the radially inward direction,while moving off the seat against spring pressure to permit flow in theradially outward direction. The configurations depicted in these Figuresare by way of examples only and it will be appreciated that other designconfigurations are possible. For example, many alternative valvingactions to induce or impart a preferential flow direction may be used,such as positive-valving or gating techniques of the diaphragm valvetype, or vortex-inducing techniques or fluid amplification techniques.

Within the general configuration shown in FIGS. 1 and 2 it is alsofeasible to establish a preferential flow direction across the mixer bymeans of placing and sizing the channels 5 and 8 at the appropriatepositions. For instance, for any expanding compartment 11 channelswithin the adjacent inner ring could be sized greater than the channelswithin the adjacent outer ring, whereas for any contracting compartment11 the converse applies. An alternative arrangement for high pumpingefficiency would be not to locate any channels within the outer ringadjacent to an expanding compartment, nor within the inner ring adjacentto a contracting compartment.

With reference to FIGS. 1 and 2 it may be noted that the radial channels5 and 8 are shown to be totally enclosed within the rotor ring 7 andeach stator ring 4 respectively. An alternative arrangement is shown inFIG. 4 in which each channel 5 and 8 is defined in the axially outeredge of respective ring 4 and 7. Each channel is not therefore totallyenclosed within its respective ring but is bounded on at least one sideby the adjacent planar surface 6 or 9. It may be noted that thesectional end-view shown in FIG. 2 is valid for FIG. 4, as are thegeneral channel formations exemplified in FIG. 3.

It may also be noted that a channel is not confined to being circular incross-section down its axis: for instance, a cross-section that iscurved but not circular, such as an oval section, or a cross-sectionthat has one or more flat or straight sides, such as a rectangularsection, are also valid as embodiments of the invention. Indeed, the useof non-circular cross-sections of the latter types may simplifymanufacture of the equipment and may also provide additional mixingbenefits such as enhanced shear stressing and extensional stressing as aresult of additional degrees of freedom being introduced into thegeometry of the channel and into the flow characteristics of thematerial within the channel.

As a further alternative modification, the channels 5 shown in FIG. 4could be formed essentially continuous in a circumferential direction soas to form an annulus, i.e. a single annular stress-inducing flowchannel (which in this embodiment is partitioned by the vanes 10).

In another example of an embodiment of the invention, FIGS. 5 and 6depict a mixing system having a predominantly axial flow, with thematerial entering at an inlet port 22 and exiting at an outlet port 23.The mixer in this example comprises a rotor shaft 24, rotationallydriven by some external means (not shown), on which two rotor discs 25are concentrically located, mounted within a housing 26 that containsthree concentrically located stator discs 27. Each rotor and stator disccontains axially-aligned stress-inducing flow channels 28, for instanceof the types shown in FIG. 3, where each channel is either totallyenclosed within its respective disc or alternatively is located withinthe circumferential surface of the disk with the inner surface of thehousing forming the enclosing surface. In this example, the stator disks27 are mounted on planes that are perpendicular to the axis of rotationof the rotor shaft 24, while the rotor disks 25 are located on planesthat are inclined with respect to the planes of the stator disk 27. Therotor disks 25 are shown to be parallel to each other, although this isnot essential and alternative arrangements are equally possible. Eachstator disk 27 contains a number of vanes 29 mounted between the outersurface of the rotor shaft 24 and the inner surface of the housing 26,and which are capable of sliding axially with respect to the surface ofthe rotor 24 and circumferentially with respect to the housing 26. Thevanes 29 extend axially from within slots located in the stator disks 27to slide against the face of the rotor disks 28.

The combination of the surfaces of rotor discs 25, stator discs 27,rotor shaft 24, housing and vanes 29 serves to enclose a set ofcompartments 30. As the rotor disks 25 rotate, each compartment becomesprogressively larger and smaller as a consequence of the non-parallelityof the rotor discs 25 relative to the stator discs 27. A pumping actionis thus provided in which material is drawn into each compartment 30 asit expands and is expelled from the compartment as it contracts. Thematerial enters and exits each compartment through the channels 28 thatare axially disposed within the adjacent discs, although a controllableamount of material flow can take place through annular spaces definedbetween the rotor disks 25 and the housing 26 and thereby provide adegree of mixing within spaces other than the channels 28.

It will be appreciated that in the geometry shown in FIG. 5 the vanes 29may or may not seal each compartment 30 along the line of sliding actionbetween each vane and an inclined surface of each rotor disc 25depending upon the construction of the individual vanes and the degreeto which circumferential transfer flow between adjacent compartments 30is desired for mixing (for instance, each vane could comprise a numberof adjacent independently slideable sections).

In operation, the material passing axially from each channel 28 issubstantially distributed in sequence, via the compartments 30 betweenstator disks 27 and rotor disks 25, to the channels 28 contained withinthe adjacent discs. The resultant dispersive and distributive mixingactions is similar to those previously described in the example ofradially-flowing mixing of FIGS. 1 and 2.

The axially-flowing mixer of FIGS. 5 and 6 thus serves to illustrate thewide range of potential embodiments of the invention, where pumpingactions are combined with mixing actions within the mixer unit. Thepumping actions are not however limited to the vane types describedwithin these examples, but can equally comprise other forms of pumpingsuch as, but not limited to, those embodying alternative means ofpositive displacement pumping, centrifugal pumping or drag-flow pumping.Indeed, with the embodiments of FIGS. 1, 2 and 4, a certain amount ofcentrifugal pumping will occur in addition to the pumping actionsdescribed above as a result of rotation of the rotor ring 7. The degreeof centrifugal pumping will depend upon the design of the mixer and thematerial being mixed and could be relatively substantial in cases of lowviscosity materials and high rotational speeds.

As an example of alternative pumping actions that may be incorporated inmixers in accordance with the present invention, the mixers of FIGS. 1,2 and 4 could readily be modified to provide centrifugal pumping only byremoving the vanes. With such an arrangement, as the rotor ring 7 isrotated, material contained within each radial flow channel would besubjected to centripetal forces which would propel the material in aradially outwards direction. A pumping action would thereby be providedin which material would be drawn from the upstream stator flow channels5 and chamber to the rotor flow channels 8 and then expelled into theouter (downstream) chamber and stator flow channels 5. There could alsobe a controllable amount of material flow through the annular spaces 12between the rotor ring 7 and the stator planar surface 6 and between thestator rings 4 and the rotor planar surface 9.

It will be appreciated that with such a centrifugal pumping mixer thematerial present in the chambers defined between the rotor ring andstator rings will be subjected to rigorous shearing actions between therotor ring 7 and the stator rings 4 and also extensional flow due to thecircumferential tapering of the chambers, in addition to the stressingthat occurs within the stress-inducing channels (the degree of stressingbeing influenced in part by the relative dimensions of the chambers).Alternatively, the stator and rotor rings could be mountedconcentrically with one another (i.e. effectively reducing the XY offsetto zero) in which case there will still be centrifugal pumping but nosignificant extensional-dispersive mixing within the chambers whichwould no longer taper (if desired vanes could be included in such anembodiment to enhance distributive mixing). As a yet furthermodification, the concentric stator and rotor rings could be sized sothat they are in sliding contact with one another.

It will be appreciated that many of the design details and operationaldetails discussed in relation to the vane-type mixers of FIGS. 1, 2 and4 apply equally to the centrifugal pumping modification discussed above.

As a further alternative, the mixers of FIGS. 1, 2 and 4 could bemodified to provide drag-flow pumping. In this case, the eccentricmounting of the stator ring 7 may be maintained but the vanes wouldpreferably be omitted. As the rotor ring 7 rotates eccentricallyrelative to the stator rings 4 the annular chambers defined therebetweenwould contain an expansion zone and a compression zone. Provided thematerial to be mixed has a sufficiently high viscosity, the drag forcesimparted to the material by the motion of the rotor 7 would besufficient to pump material out from the compression zone through theadjacent radial flow channels. With an arrangement similar to that shownin FIG. 2, a series of alternating compression and expansion zones areeffectively provided in any radial direction. Given that the radial flowchannels 5 and 8 are capable of biasing the radial flow in favour of onedirection over the other, a net flow of material through the mixer wouldbe obtained.

Alternative constructions utilising other pumping mechanisms (orcombinations of pumping mechanisms) could readily be constructed by theappropriately skilled person.

It may be noted that the examples of the mixers shown depict a limitednumber of mixing stages. It is an aspect of the present invention thatmore than one stage of mixing may be provided by means of additionalrotor and stator stages, or that less stages could be provided by, forexample, by reducing the number of rings to one stator ring and onerotor ring. For example, the radially-flowing mixer shown in FIG. 1comprises two stator rings and one rotor ring. To this number may beadded a further number of rotor and stator rings, where each stator ringis generally concentric with the other stator rings and lies on the sameplane of the stator disk, where each rotor ring is generally concentricwith the other rotor rings and lies on the same plane of the rotor disk,and where the rotor rings and the stator rings form alternate layers inthe radial direction in the general manner shown in FIG. 1. Anotherexample can be taken with reference to FIG. 4, where two rotor rings andthree stator rings are shown. Additional numbers of rotor and statorrings can be added to the unit at locations along the axis of rotationof the rotor, where rotor disks and stator disks are alternately locatedalong the axis.

It is thus shown that the invention allows for a number of mixing stageswithin a single mixing unit. It is another aspect of the invention thatany individual stage need not contain the same volume of material as anyother stage. This variation in volume is exemplified in FIG. 2, wherethe volume of material contained within the annular chambers definedbetween successive rings increases in the radial direction as aconsequence of the increasing diameter of the rings. This feature is ofimportance when considering the performance of the mixing system inoperating with additional streams of material, such as dilutant fluids,that are introduced into the mixing system after or before specificstages in the mixing operation. For instance, in a multi-stage mixer ofthe type shown in FIG. 1, the first stage could be used to achieve someinitial mixing of the materials that entered the mixing system throughthe inlet, whereas the addition of material at an injection point 31located within the second stage would permit such material to be mixedtogether with the initially mixed material and passed to the outletport. The feature of expanding volumes of subsequent stages enables themixing system to cope with increasing volumes of material withoutsignificantly altering the individual mixing actions that the materialis being subjected to in successive mixing stages.

As an alternative aspect of the ability of the mixing system to providevolumes that differ from stage to stage, the reverse situation can beapplied to the radially-flowing mixing system described in the precedingparagraph, namely that a flow in the reverse direction, that is radiallyinwards, can be used in situations such as those in which material is tobe extracted from the mixer at intermediate stages. In this it should benoted that the reversal of the flow direction would be achieved byreversing the pumping effects by means of reversing the orientation ofthe channels previously described.

It is also a feature of the present invention that the pumping andmixing performances of an individual mixing unit can be varied before orduring operation by means of adjusting the rotational speed of the rotoror the geometry of the mixer unit, more specifically the geometricalrelationship of rotor to stator. For example, the amount of eccentricityof rotor to stator in the radial-flowing mixer of FIGS. 1 and 2 affectsthe pumping rate and hence the mixing effectiveness: this eccentricitycan be set permanently, thereby establishing the ultimate performance ofthe mixer, or temporarily, in which the relative pumping performance andhence mixing performance can be set. In the example shown in FIG. 2,this temporary adjustment would require the axis of the rotor to bemoved closer towards the axis of the stator, thereby reducing thepumping effectiveness of the unit but, for example, possibly enhancingits distributive mixing capability. In the example shown in FIGS. 5 and6, the variations to performance could similarly be achieved by alteringthe inclination of the rotor disks with respect to the stator disks.

The invention has application in all areas of fluid mixing and acrossall industries where mixing is required, for example the chemical, food,healthcare, medical, petrochemical and polymer industries. The inventionalso has application in areas of solids mixing where such solids can beconsidered to respond to the imposed forces in an essentially fluid-likemanner, or where the solids are fragmented to the extent that, in theaggregate, they are capable of behaving in a manner analogous to fluids,or any combination of fluids and solids.

What is claimed is:
 1. A mixing apparatus for mixing a material, theapparatus comprising one or more stress inducing flow channels and atleast two members eccentrically mounted one within the other so as todefine a chamber therebetween, and which are rotateable relative to oneanother to thereby produce a pumping force to force material throughsaid flow channels and chamber to thereby subject the material tostresses within at least one of said flow channels and said chamber thatresults in at least an extensional-dispersive or a shear-dispersivemixing.
 2. A mixing apparatus according to claim 1, comprising aplurality of flow channels provided in at least two sets defined byrespective channel members at least one of said flow channels being asaid stress inducing flow channel, the channel members being arrangedsuch that material is pumped from the or each flow channel of one set tothe flow channel or channels of another.
 3. A mixing apparatus accordingto claim 2, comprising more than two sets of said flow channels arrangedto receive material in sequence, wherein a pumping force is imparted tothe material between each set of flow channels.
 4. Apparatus accordingto claim 2, wherein each set of flow channels comprises a plurality ofstress-inducing channels.
 5. Apparatus according to claim 4, wherein theflow channels are arranged such that material pumped from each channelof one set of flow channels is distributed between a number of flowchannels of another set of flow channels to achieve distributive mixing.6. Apparatus according to claim 5, wherein the channels are arrangedsuch that each channel of one set of flow channels receives materialfrom a plurality of flow channels of another set of flow channels tofurther promote interleaving and distributive mixing of the material. 7.Apparatus according to claim 2, comprising a plurality of pairs of saidchannel members, each pair defining a respective chamber therebetween,wherein the volumes of the respective chambers vary between successivepairs of channel members to permit material to be drawn off from, oradded to, the mixing apparatus at intermediate stages of mixing withoutadversely affecting the pumping performance of the apparatus. 8.Apparatus according to claim 2, wherein the volume of said flow channelsvaries between successive pairs of channel members to permit material tobe drawn off from, or added to, the mixing apparatus at intermediatestages of mixing without adversely affecting the pumping performance ofthe apparatus.
 9. A mixing apparatus according to claim 2, comprising aplurality of pairs of said channel members each pair defining arespective chamber therebetween, and one or more partition membersextending between adjacent channel members to partition the respectivechamber, wherein a space is defined between the or each partition memberand an adjacent wall of a channel member, and wherein the size of saidspaces varies between successive chambers defined between successivepairs of channel members to permit material to be drawn off from, oradded to, the mixing apparatus at intermediate stages of mixing withoutadversely affecting the pumping performance of the apparatus.
 10. Amixing apparatus according to claim 2, wherein one or more channels ofeach set of flow channels are defined partly by said respective channelmember and partly by another member which may be an adjacent channelmember.
 11. A mixing apparatus according to claim 2, wherein at leastone of said at least two members comprises one of said channel members.12. A mixing apparatus according to claim 2, wherein the at least twochannel members are positioned relative to one another so as to definesaid chamber therebetween, said chamber comprising at least onecompartment through which the material passes between two sets of flowchannels, wherein in operation the volume of the or each compartmentsuccessively increases and decreases to provide positive displacementpumping.
 13. A mixing apparatus according to claim 12, wherein one ofsaid at least two channel members is mounted for rotation relative tothe other, the rotating channel member carrying one or more partitionmembers extending between said at least two channel members to partitionsaid chamber and define said at least one compartment such that said atleast one compartment rotates with the rotating channel member, andwherein the geometry of said chamber is such that the volume of said atleast one compartment progressively increases and decreases as itrotates, the volume of said at least one compartment being determined byits angular position.
 14. A mixing apparatus according to claim 13,wherein the at least two channel members are at least substantiallyannular with successively increasing diameters and are arranged suchthat one channel member surrounds another.
 15. A mixing apparatusaccording to claim 14, wherein the at least two channel members arearranged in one or more pairs, the channel members of the or each pairbeing eccentrically mounted relative to one another.
 16. A mixingapparatus according to claim 14, wherein said at least two channelmembers are arranged in one or more pairs, one channel member of the oreach pair of channel members being fixed in position.
 17. A mixingapparatus according to claim 14, comprising at least three of saidchannel members two of which are concentric and fixed in position andthe third of which is mounted for eccentric rotation between the othertwo.
 18. A mixing apparatus according to claim 14, comprising at leastthree of said channel members two of which are concentric and rotateabout a third channel member which is fixed in position between them.19. A mixing apparatus according to claim 12, wherein at least some ofsaid flow channels are configured to favor flow in a downstreamdirection such that on alternation of the direction of pumping toprovide backflow there remains a net downstream flow.
 20. A mixingapparatus according to claim wherein at least some of said flow channelsare provided with valve means which favor flow in the downstreamdirection.
 21. A mixing apparatus according to claim 12, wherein one ofsaid at least two channel members is mounted for rotation relative tothe other which does not rotate, the non-rotating channel membercarrying one or more partition members extending between said at leasttwo channel members to partition said chamber and define said at leastone compartment, and wherein the geometry of said chamber is such thatthe volume of said at least one compartment progressively increases anddecreases as the rotating channel member rotates, the volume of said atleast one compartment being determined by the angular position of therotating channel member.
 22. Apparatus for mixing a material, theapparatus comprising one or more flow channels defined by each of atleast two channel defining members which are axially mounted within ahousing, at least one of said flow channels being a stress-inducing flowchannel, at least one of said channel members being rotateable on ashaft which extends to the other such that a chamber and an enclosingvolume is defined between the two channel members around said shaft andan inner surface of said housing, one of the channel members beingprovided with one or more partition members extending between the twochannel members to partition said chamber into two or more compartments,and wherein said channel members have non-parallel facing surfaces suchthat as the rotating channel member rotates, a respective volume of eachcompartment progressively increases and decreases as a function of itsannular position about said shaft.
 23. Apparatus according to claim 22,wherein said channel members are substantially disc shaped and adjacentchannel members are angled relative to one another to produce saidnon-parallel faces.
 24. Apparatus according to claim 22, comprising atleast three channel members mounted about said shaft, two of saidchannel members being fixed in position and a third being rotatablebetween the other two.
 25. Apparatus according to claim 22, wherein saidhousing is generally cylindrical such that the or each chamber isdefined in part by a wall of the housing.
 26. Apparatus according toclaim 22, wherein each channel member defines a plurality of said stressinducing channels.
 27. Apparatus according to claim 26, wherein saidflow channels are arranged such that material pumped from each channelof a first of said channel members is distributed between a plurality ofchannels of a second of said channel members to thereby achievedistributive mixing.
 28. Apparatus according to claim 27, wherein theflow channels are arranged such that each channel defined by a first ofsaid channel members receives material from a plurality of channelsdefined by a second of said channel members to further promoteinterlieving and distributive mixing of the material.
 29. Apparatusaccording to claim 22, comprising a plurality of pairs of said channelmembers defining respective chambers therebetween, wherein the volumesof the respective chambers vary between successive pairs of channelmembers to permit material to be drawn off from, or added to, the mixingapparatus at intermediate stages of mixing without adversely affectingthe pumping performance of the apparatus.
 30. Apparatus according toclaim 22, wherein the volume of said flow channels varies betweenadjacent channel members to permit material to be drawn off from, oradded to, the mixing apparatus at intermediate stages of mixing withoutadversely affecting the pumping performance of the apparatus. 31.Apparatus according to claim 22, wherein said flow channels areconfigured to favor flow in a downstream direction such that onalternation of the direction of pumping to provide backflow thereremains a net downstream flow.
 32. Apparatus according to claim 31,wherein at least some of said flow channels are provided with valvemeans which favor flow in said downstream direction.
 33. Apparatusaccording to claim 22, comprising at least three channel members mountedabout said shaft, two of said channel members being rotatable about athird which is fixed to said shaft.